The present invention relates to enameled wires having great resistance to overload and more particularly, to enameled wires that have great resistance to overload and which are not easily shorted even if an overcurrent flows into the motor coils.
Enameled wires used in electric and electronic machines are subject to high temperatures due to the Joule's heat generated by current flow which produces a magnetic field. Accordingly, such wires must be provided with heat resistance that prevents the deformation and thermal deterioration of the wire enamel coating. With the use of various heat-resistant polymeric materials to make the wire enamel coat of electric wires, the size of the machines have been reduced and their reliability has been increased due to fewer shortings. This shows how important it has been to improve the heat resistance of the wire enamel coating of electric wires.
These days, the stationary or mobile coils in electric automotive parts or motors used in especially high-temperature atmospheres as in chemical plants must perform well under hostile conditions such as in high temperature atmospheres, much hotter than conventional ones. More specifically, it is required that no shorting occur between enameled wires and that the coil retain its ability to produce a magnetic field even in hot atmospheres with mechanical vibrations or even if an abnormally great overcurrent flows into the wires.
To meet this requirement, there have been proposed electric wires with a wire enamel coat that becomes ceramic in hot atmospheres. The wires are made of a conductor having a baked layer of a wire enamel comprising a silicone resin and an inorganic material. The silicone resin may be selected from among all types of silicone resin including modified types. Suitable inorganic materials are fine particles of alumina (Al.sub.2 O.sub.3), barium titanate (BaTiO.sub.3), zircon (ZrSiO.sub.4), calcium titanate (CaTiO.sub.3), lead titanate (PbTiO.sub.3), barium zirconate (BaZrO.sub.4), steatite (MgSiO.sub.3), silica (SiO.sub.2), beryllia (BeO), zirconia (ZrO.sub.2), magnesia (MgO), clay, kaolin, bentonite, montmorillonite, glass frit, talc, mica, boron nitride, silicon nitride, pyropyllite, aluminum, zinc, nickel, etc.
These wires are made by applying a wire enamel comprising these silicone resins and inorganic materials onto a conductor and baking. These wires can be used equally well as conventional enameled wire under normal conditions and, if the temperature becomes abnormally high, the wire enamel becomes ceramic allowing for normal operation of the machine to continue at high temperatures.
These enameled wires, however, have the following tow defects.
(1) The wire enamel is damaged during coiling operations which subject the wire to mechanical stresses such as elongation, compression, friction and twisting. If damaged, the resulting coil does not have the desired resistance to overload. It is, of course, possible to reduce mechanical damage by using an organic wire enamel that has high flexibility and mechanical strength which has conventionally been employed as a topcoat. However, even with this construction, it is often not possible to prevent significant reduction in the physical properties of the coat after coiling. For instance, a wire that is elongated by 3 to 20% has a resistance to overload about 40 to 70% lower than that of an unextended wire.
(2) A wire enamel that has become ceramic under high temperature atmospheres cracks due to heat cycles accompanying the subsequent cyclic operation of the machine. The cracked enamel will peel off the conductor causing shorting.